fix #H4428 - mon vs mon thrown cockatrice egg

There was no code in ohitmon() (for object thrown or launched at a
monster by someone or something other than the hero) to handle an egg
hitting a monster.  Cockatrice egg is monsters' preferred missile,
but if one hit a monster instead of hero it just did minimal damage
without any chance of the side-effect that makes it be preferred.

doc/fixes36.1

@@ -316,6 +316,8 @@ when confused scroll of light summoned lights, player would be asked what to
 	call the scroll even if scroll of light was already identified
 if a visible monster becomes invisible, mark its spot with the 'remembered,
 	unseen monster' glyph ('I' character or '?' tile)
+monsters can throw cockatrice eggs at hero, but there was no handling for eggs
+	when the missile hit an intervening monster
 
 
 Fixes to Post-3.6.0 Problems that Were Exposed Via git Repository